The backbone 18 is intended for transmission of information among the BS1 11, the BS2 13, the BS3 15 and the central processing unit 19.
The central processing unit 19 can be used to support the BS1 11, the BS2 13, and the BS3 15 perform pre-coding of data.
FIG. 2 shows a flow chart 38 for a method of using the telecommunication system 10.
The method comprises selecting the first air communication path 30 and the second air communication path 31, as illustrated a step 40 of in FIG. 2.
The selection is done such that the first interference path 34 has a low power across operative frequency range. The low power here implies that channel coefficients of the first interference path 34 have low values or a power that is low with reference to average signal power. The other interference path 35 has high power. The high power implies a power that is higher that the low power.
The high power here refers a power that is higher than the low power.
The Tx2 21 then signals the Rx2 26 to measure channel coefficients of the second interference path 35. The channel coefficients of the second interference path 35 and the second air communication path 31 are afterward measured, as shown in a step 42 of FIG. 2.
The channel coefficients include channel path gains and phase. Methods to estimate channel coefficients include channel interpolation, adaptation to time variance, and frequency selectivity of transmission channels or advanced scheme, such as subspace adaptation.
The significant channel coefficients of the second interference path 35 are later identified and they are then reported to the BS2 13, as depicted in a step 44 of FIG. 2. Selection criteria of the significant channel coefficients are large values and that they correspond to channel coefficients of the second interference path 35 that also has large values. Since only significant channel coefficients are reported, the reporting is selective thereby producing a reduced number of feedback.
The BS2 13 then obtains of next shared relevant frequency resource information from the BS1 11. This is shown in a step 46 of FIG. 2. The next shared relevant frequency resource information includes information about the time slots in which the Tx1 20 is scheduled to send first data to the Rx1 25 with the same frequency resource as the Tx2 21. The second BS2 13 also obtains from the BS1 11 data format information of the first data that is intended to be send form the BS1 11 to the UE1 12. The data format information includes coding, interleaving, and modulation mapping. The shared relevant frequency resource information or the data format information may be called user data.
The BS2 13 later performs non-linear spatial pre-coding on the second data that is present on the shared frequency resource before transmission of the second data, as depicted in a step 48 of FIG. 2. The BS2 13 applies Costa's scheme of pre-coding on the second data to reduce cross talk.
The non-linear pre-coding methods is be implemented using Costa's scheme, “Writing on dirty paper”, or a variant of the Costa's scheme. The Costa's scheme can effectively reduce signal degradation due to interference from cross talk. This scheme is shown in M. Costa, “Writing on dirty paper”, IEEE Transactions on Information Theory, vol. 29, pp. 439-441, May 1983.
The Costa's scheme provides an adjustment of a signal, before the signal is transmitted, to account for interference that the signal will encounter during transmission. In this scheme, pilot symbols of interference source and of interference channel are known prior to transmission of the signal.
Overall channel can be decomposed by a QRD (QR Decomposition) or a QLD (QL Decomposition) technique into a triangular structure of a corresponding equivalent channel.
The Tx1 20 then sends the first data to the Rx1 25 and the Tx2 21 then the second data to the Rx2 26, as shown in a step 50 of FIG. 2.
Advantages of the method are described below.
The method can be combined with user grouping methods in order to benefit from right selection of user pairs that are suitable for the application.
The method works well with top-M feedback approach that is discussed in 3GPP LTE (Long Term Evolution).
Channel coherence time of signal and interference power usually strongly depends on signal strength and interference strength. Deep signal fade and interference fade generally have a much shorter duration than the above average signals. Therefore, the channel coherence time for selected strong channels is significantly longer than for those where a good signal to interference ratio is due to a fade on interfering channel coefficient.
Channel estimation of a strong interfering signal is more accurate than channel estimation of a fade.
The UE2 14 reports on resources that meet described selection criteria. This reduces channel coefficient feedback signalling to only one channel coefficient per frequency resource or resource block and to only the more significant resource. This results in a substantial feedback reduction.
Information exchange for coordinated non-linear pre-coding reduces to selective feedback of a few channel coefficients and to an exchange of user data between the BS1 11 and the BS2 13 prior to transmission of signal.
Non-linear pre-coding, such as Costa's “Writing on Dirty paper” scheme, can reduce or eliminate influence of interference, if the interference channel and interfering data symbols are known prior to transmission of signal.
Calculations of pre-coding algorithms required of the method are less than the corresponding calculations required for a full-cooperation system. Most pre-coding algorithms, such as Zero forcing (ZF), generate mutual interference nulls between receiving mobile stations. Due to large spatial separation of transmission antennas, these interference nulls can be unstable, which require a high update rate of channel state information (CSI) information.
When performing non-linear pre-coding, instead of non-linear decoding, only binary data including coding, interleaving, and modulation mapping need to be known. Whilst non-linear detection in general requires an exchange of quasi-analogue samples at symbol level that requires higher data rates between the two BS and more stringent timing requirements.
Temporal variation of the transmitting signal and interference from neighbouring transmit antenna only depend on mobility of the UE2 14 and not on the mobility of the UE1 12. When this method is applied on the uplink, the interference channel is determined by the mobility of the UE1 12 as well and is not independent of the UE1 12 anymore.
In a broad sense, the method can be applied for an uplink transfer of signal as well as downlink transfer of the signal.
The BS1 11, the BS2 13, or the BS2 15 can transmit or receive signals from the UE1 12, the UE2 14, or the UE3 16, respectively.
The method may include a step of compensating interference from more than one source, such as the BS3 15. The step includes selecting a suitable user grouping, the user group comprises a plurality of links. Interference of all interference channels are ranked for the link. The interference is based on channel coefficients of the interference channel. Accordingly, more data from the receive antenna have to be requested from interfering base stations and from the interference channels.
The method can also be applied to multi-carrier systems, such as OFDM (Orthogonal Frequency-Division Multiplexing), DFT (Discrete Fourier Transform)-pre-coded OFDM, MC-CDMA (Multi-Carrier Code Division Multiple Access) and also standard CDMA (Code division multiple access), as long as triangular structure of composed multi-user multi-BS channel can be obtained by suitable user grouping.
The method can apply other forms of non-linear spatial pre-coding, such as Tomlinson-Harashima pre-coding and vector perturbation techniques.
The Tomlinson-Harashima pre-coding is shown in M. Tomlinson, “New Automatic Equalizer Employing Modulo Arithmetic”, Electronic Letters, pp. 138-139, March 1971, and
H. Harashima, Miyakawa, “Matched-Transmission Technique for Channels with Intersymbol Interference”, IEEE Journal on Communications, pp. 774-780, August 1972.
The vector perturbation technique is shown in
B. Hochwald, B. Peel, and A. L. Swindlehurst, “A Vector perturbation technique for near capacity multiantenna multiuser communication-Part II: Perturbation,” IEEE Trans. Comm., vol. 53, no. 3, pp. 537-545, March 2005.
In short, the above method starts with selecting a first link that is placed between a first transmit antenna and a first receive antenna and a second link is placed between a second transmit antenna and a second receive antenna. The first transmit antenna is for sending a first data to the first receive antenna whilst the second antenna is for sending a second data to the second receive antenna.
The first link has a first cross-channel that has low power over operative frequency range. The first cross-channel is being placed between the second transmit antenna and the first receive antenna. The second cross-channel is placed between the first transmit antenna and the second receive antenna.
After this, second channel coefficients of a second cross-channel and the second link are measured at the second receive antenna. Significant second channel coefficients are then reported to second base station of the second transmit antenna. The significant channel coefficients of the second link have large values and they correspond to channel coefficients of the second cross channel that also has large values.
The second base station then obtains next relevant shared frequency resource information from a first base station of the first transmit antenna. The next shared frequency resource information includes time slots that the first transmit antenna is scheduled to send user data to the first receive antenna at the same frequency resource as the second transmit antenna. The second transmit antenna is also aware of corresponding data of the user data, such as coding, interleaving, and modulation mapping.
The second base station afterwards applies non-linear spatial pre-coding for the second data before transmission of the second data to reduce cross talk. After this, first base station sends the first data and the second base station sends the pre-coded second data on the shared frequency resource.
List of Abbreviations
                BBS Broadcasting Base Station        BCG Broadcast Cellular Guard        BS1 first base station        BS2 second base station        BS3 third base station        BWA Broadband Wireless Access        CSI channel state information        FDD frequency division duplexing        MIMO multiple input multiple output        Rx1 first receive antenna        Rx2 second receive antenna        Rx3 third receive antenna        Tx1 first transmit antenna        Tx2 second transmit antenna        Tx3 third transmit antenna        UE1 first user equipment        UE2 second user equipment        UE3 third base stationReference Number List            10 telecommunication system    11 first base station (BS1)    12 first user equipment (UE1)    13 second base station (BS2)    14 second user equipment (UE2)    15 third base station (BS3)    16 third user equipment (UE3)    18 backbone    19 central processing unit    20 first transmit antenna Tx1    21 second transmit antenna Tx2    23 third transmit antenna Tx3    25 first receive antenna Rx1    26 second receive antenna Rx2    27 third receive antenna Rx3    30 first air communication path    31 second air communication path    32 third air communication path    34 first interference path    35 second interference path    38 flow chart    40 step    42 step    44 step    46 step    48 step    50 step